Method and circuitry for causing sixteen bit microprocessor to execute eight bit op codes to produce either internal sixteen bit operation or internal eight bit operation in accordance with an emulation bit

Abstract

A sixteen bit microprocessor includes an emulation bit that is loaded into an instruction register with 8 bit op codes. If the emulation bit is a "1", the 16 bit microprocessor operates internally as a 6502 type eight bit microprocessor, thereby emulating a 6502 microprocessor. If the emulation bit is set to a "0", the 16 bit microprocessor executes an instruction set of which the 6502 instruction set is a subset to effectuate internal 16 bit operation.

Description

BACKGROUND OF THE INVENTION

The invention relates to an efficient topography for a sixteen bit CMOS microprocessor chip having the capability of either operating as a sixteen bit microprocessor or operating to emulate the well-known 6502 eight bit integrated circuit microprocessor, depending only on the state of a software "emulation bit" or "E" bit.

Those skilled in the art of integrated circuit design, and particularly microprocessor chip design, know that the size of a high volume integrated circuit chip is a dominant factor in the ultimate yield and manufacturing cost per unit. In all of the integrated circuit technologies, including the CMOS technology, large scale integrated (LSI) chips such as microprocessor chips include thousands of conductive lines and P-channel MOSFETS (metal-oxide-semiconductor field effect transistors) and N-channel MOSFETS. Some of the lines are composed of aluminum metal interconnection layers and others are composed of polycrystalline silicon interconnection layers on different insulative layers, and others of the conductors are diffused conductors. Certain minimum line widths and spacings between the respective lines and the sources and drains of the MOSFETS must be maintained to avoid short circuits and parasitic effects despite slight variations in the manufacturing processes due to presence of minute particulates that are invariably present in the semiconductor manufacturing facilities. Due to the low current supplying capability of the very small MOSFETS that must be used in order to achieve high functional density of the integrated circuits, the lengths of the interconnecting lines and their associated capacitances must be minimized, not only to reduce chip size, but also to achieve maximum circuit operating speeds. A wide variety of design trade-offs, including the necessity to minimize chip size, obtain a suitable chip aspect ratio (which enhances integrated circuit chip yield and wire bonding yield), increase circuit operating speeds, reduce power consumption, and achieve acceptable reliability all are involved in obtaining an optimum "layout" or topography of the MOSFETS and the interconnection patterns therebetween are required in order to obtain an integrated circuit which is both economical and has acceptable operating characteristics.

Some of the numerous design constraints faced by the MOSLSI chip designer include specifications for minimum widths and spacings of diffused regions in the silicon, minimum widths and spacings for metal interconnection lines, the minimum size required for polycrystalline silicon conductors, the minimum size required for contact openings in the insulating "field" oxides, the spacings required between the edges of contact openings to the edges of the diffused regions or polycrystalline silicon regions, the fact that polycrystalline silicon conductors cannot cross over each other or over diffused regions in most silicon gate manufacturing processes, and the constraint that conductors on the same layer of insulating oxide cannot cross over like conductors.

Furthermore, the high amount of capacitances associated with diffused regions and the high resistances of both diffused regions and polycrystalline silicon conductors must be carefully considered by the circuit designer and also by the chip designer in arriving at an optimum chip topography. For many types of circuits, such as the microprocessor of the present invention, an extremely large number of conductive lines between sections of logic circuitry are required. The practically infinite number of possibilities for routing the various conductors and placing of the various MOSFETS taxes the skill and ingenuity of even the most resourceful chip designers and circuit designers (and is far beyond the capability of the most sophisticated computer layout programs yet available). Other constraints faced by the chip layout designer and the circuit designer involve the need to minimize cross-coupling and parasitic effects which occur between various conductive lines and conductive regions. Such effects may degrade voltages on various conductors, leading to inoperative circuitry or low reliability operation under certain operating conditions.

The technical and commercial success of an electronic product utilizing MOSLSI technology hinges on the ability of the chip designer to achieve an optimum chip topography. It is well known that a very high level of creative effort is required, usually both by circuit designers or layout draftsmen, to achieve a chip topography or layout which enables the integrated circuit to have acceptable operating speeds and low power dissipation and yet is sufficiently small in chip area to have a high chip per wafer yield, i.e., to be economically feasible. Often, many months of such effort between chip designers and circuit designers result in numerous trial layout designs and redesigns and concomitant circuit design revisions before a reasonably optimum topography for a single MOSLSI chip is achieved. Often, until a particular new overall insight or approach is conceived for a particularly difficult, complex, and large functional subsection of an integrated circuit chip, such as an instruction decoding subsection in a microprocessor, is arrived at, the desired chip is clearly economically unfeasible. It is on the basis of such an insight that the eight bit CMOS chip topography described in the above-mentioned parent application became economically feasible, resulting in a CMOS that has become a commercial success. It is on the basis of another such insight, arrived at after over a year of design and layout experimentation, that the sixteen bit CMOS microprocessor layout of the present invention could be reduced to a size that made the chip size commercially practical and resulted in adequate operating speed.

Although various single chip microprocessors, such as the 6502, the 6800, the 68000, the 8088, the Z80, and others have been widely used, all of them have various shortcomings from the viewpoint of a computer system designer trying to design a low cost computer system because of the inconvenience of implementing certain functions. Some of the functions that are difficult to implement using prior single chip microprocessors include the problems of efficiently using program memory, i.e., typically slow ROM (read-only memory), in which the program is stored and data memory, i.e., high speed memory in which intermediate data results are stored, dealing with abort conditions such as invalid addresses, which requires aborting the present instruction, executing an abort subroutine, and then re-executing an entire subroutine in which the aborted instruction was contained.

Another problem that is faced by computer designers using prior single chip microprocessors is the need to have new software written for computers that use newly developed, faster microprocessors with greater computing power. It would be very desirable to provide a technique by means of which newer microprocessors, such as 16 or 32 bit microprocessors, can execute already available software written for previous single chip microprocessors having fewer bits in their data words, fewer instructions, and generally less computing capability.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide an economical topography for a CMOS sixteen bit microprocessor chip.

It is another object of the invention to provide an economical topography for the layout of a sixteen bit CMOS microprocessor chip capable, under software control, of either emulating the prior well-known 6502 eight bit microprocessor or operating as a sixteen bit microprocessor having advanced abort operation and other advanced capabilities.

It is another object of the invention to provide an improved sixteen bit CMOS microprocessor chip that incorporates various new features which provide a system designer increased flexibility in dealing with memories of different speeds, in dealing with interrupts of various priorities, and in recovering from an abort condition.

Briefly described, and in accordance with one embodiment thereof, the invention provides an efficient topography for an integrated circuit sixteen bit CMOS microprocessor chip including emulation logic circuitry for enabling the microprocessor, under software control, to emulate a pre-existing eight bit microprocessor or to operate as a sixteen bit processor with advanced operating capabilities, the chip including a surface with left, bottom, right, and top edges, the chip including on its surface a data bus, an address bus, low order address buffers located along the left lower side, high order address buffers located along the bottom edge, data bus buffers located on the lower right edge, a relatively large register section including low order address latches, arithmetic logic unit circuitry, stack registers, program increments and latches, high order address latches, status register circuitry, data output latches, and data input latches positioned in the foregoing sequence from the left side to the right side of the chip, and located generally above the high order address buffers, and between the low order address buffers and the data bus buffers the chip also including instruction register circuitry and predecode circuitry located along the right edge of the chip above the data bus buffers, timing control circuitry located along the left edge above the low order address buffers, N-channel minterm read-only memory decoding circuitry including minterms that result from a first level of decoding of op codes for sixteen bit microprocessor operation, minterm inverter drivers disposed beneath the N-channel minterm read-only memory decoding circuitry, and N-channel "sum-of-minterm" read-only memory decoding circuitry disposed between the minterm inverter drivers and the register section. In the described embodiment of the invention, the N-channel minterm read-only memory decoding circuitry includes a plurality of "vertical" diffused lines, a plurality of polycrystalline silicon gate conductors defining the minterms being decoded, and a plurality of "horizontal" metal conductors connected to outputs of the instruction register and responsive to certain bits, including an emulation bit of the status register, which horizontal metal conductors make contact to predetermined ones of the polycrystalline silicon gates. The emulation bit of the status register is coupled to emulation output logic circuitry located in the upper right-hand corner of the chip and is also connected to auxilliary index register circuitry and stack register circuitry in the register section. Specialized circuitry located along the top edge of the chip from left to right includes non-maskable interrupt logic, memory lock logic, maskable interrupt logic, abort logic, ready logic, vector pull logic, reset interrupt logic, valid data address logic, status register logic, clock oscillator circuitry, and data bus enable circuitry adjacent to the upper boundary of the N-channel minterm circuitry. Valid program address logic is located above the low order address buffers along the left edge of the chip. A conductor from the abort logic is connected to the auxilliary index registers, stack registers, accumulator circuitry and status register circuitry, i.e., to all registers that the programmer has access to, to inhibit transfers of data from any of those registers during an abort operation.

In the N-channel sum-of-minterm read-only memory decoding circuitry, a plurality of vertical diffused lines are connected to a ground conductor and have various short horizontal extensions thereof at locations where field effect transistors (FETs) are required in the sum-of-minterm decoding circuitry. A plurality of vertical polycrystalline silicon conductors are connected to the outputs of the respective minterm inverter drivers and run parallel to and the respective vertical vertical diffused lines, intersecting the various horizontal extensions thereof to define the N-channel FETs. Sum-of-minterm outputs are produced on a plurality of horizontal metal conductors extending through the sum-of-minterm read-only memory decoding circuitry and making electrical contacts to various ones of the diffused drain regions of the various N-channel FETs. The horizontal metal lines to which the drains of only a few of the sum-of-minterm FETs are connected are positioned at the bottom of the sum-of-minterm read-only memory decoding circuitry, and diffused or polycrystalline silicon conductors from register transfer logic of the register section are connected to appropriate ones of those lower horizontal metal conductors. Those horizontal metal conductors having the largest numbers of sum-of-minterm field effect transistors connected thereto are located near the top of the sum-of-minterm read-only memory decoding circuitry, and are routed around the right end of that circuitry, downward, and are extended by means of diffused or polycrystalline silicon conductors as needed to conduct the sum-of-minterm signals to appropriate portions of the register transfer logic. Horizontal metal conductors having intermediate numbers of FETs connected thereto are located generally in the middle of the sum-of-minterm read-only memory decoding circuitry, and are arranged to allow diffused or polycrystalline silicon conductors to extend downward into the register transfer section.

In accordance with another embodiment of the invention, the invention provides a microprocessor chip which contains an emulation bit that causes the microprocessor chip to emulate a different microprocessor that internally operates on data words having fewer bits by using the emulation bit to "extend" op codes of the different microprocessor to generate register transfer signals that effectuate operation in an "emulation" mode or a "native" mode. In the described embodiment of the invention, the sixteen bit microprocessor emulates a pre-existing eight bit microprocessor if the emulation bit is a "one". If the emulation bit is a "one", this forces two other bits to be logical "ones". The additional bits cause certain indexing operations to be eight bit operations, and cause operations on the stack registers to be sixteen bit operations, and modifies operations on certain status register bits. The emulation bit and the two additional bits also are used in conjunction with decoding the eight bit op codes. Abort logic circuitry is provided which responds to an external signal indicating an abort condition and prevents modification of data in all registers accessible by the programmer, including the program counter latches, the index registers, the stack registers, the accumulator registers, the status register, and several other registers. After the abort circuitry has been reset, execution of the instruction during which the abort condition occurred can be repeated using the information in the registers at the time the abort condition occurred. Circuitry is provided for generating valid program address and valid data address signals which facilitate convenient accessing and operation of relatively fast data memory and relatively slow program memory. Vector pull circuitry is provided to generate an external signal that indicates when an interrupt vector is being generated, facilitating, in some cases, simple interrupt prioritizing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the general location of major circuit sections on the sixteen CMOS microprocessor chip of the present invention.

FIG. 2 is a schematic circuit diagram illustrating the technique for interconnection of diffused, polycrystalline silicon, and metal conductors in the sum-of-minterm decoding region of the microprocessor chip of the invention.

FIG. 2A is a diagram illustrating the circuitry in the minterm, minterm inverter driver, and sum-of-minterm and mode select decoding portions of the microprocessor chip.

FIG. 3A is a partial layout illustrating the connection of diffused minterm lines, polycrystalline silicon gate conductors, and metal control lines in the minterm decoding region of the microprocessor chip.

FIG. 3B is a diagram illustrating a partial layout of the technique for layout of diffused lines, N-channel gate electrodes and metal jumpers in the microprocessor layout described in the above-referenced parent application.

FIG. 4 is a block diagram illustrating the architecture of the microprocessor chip.

FIG. 5 is a scale negative image of a photomask used to pattern the interconnect metal during manufacture of the sixteen bit CMOS microprocessor chip of the present invention, with major sections shown in FIG. 1 being blocked out in heavy lines.

FIG. 6 is a scale negative image of a photomask used to define diffusion regions during the manufacture of the sixteen bit CMOS microprocessor chip of the present invention.

FIG. 7 is a scale negative image of a photomask defining regions in which N-channel MOSFETS can be produced on the microprocessor chip of the present invention.

FIG. 8 is a scale positive image of a photomask used in defining the polycrystalline silicon layer of the CMOS microprocessor chip of the present invention.

FIG. 9 is a scale positive image of a photomask used in defining the N-type diffused regions in the CMOS microprocessor chip.

FIG. 10 is a scale positive image of a photomask used in defining the P-type diffused regions in the CMOS microprocessor chip.

FIG. 11 is a scale positive image of a photomask defining all metal-to-diffusion, metal-to-polycrystalline silicon, and polycrystalline silicon-to-diffusion contacts in the microprocessor chip.

FIG. 12 is a scale negative image of the photomask used in defining the metalization pattern of the sixteen bit CMOS microprocessor chip.

FIG. 13 is a logic diagram illustrating the emulation circuitry generally indicated in FIGS. 1 and 4.

FIG. 14 is a logic diagram illustrating the abort circuitry generally indicated in FIGS. 1 and 4.

FIG. 15 is a logic diagram illustrating the valid program address (VPA) circuitry of FIGS. 1 and 4.

FIG. 16 is a logic diagram of the valid data address (VDA) circuitry indicated in FIGS. 1 and 4.

FIG. 17 is a logic diagram of the vector pull (VP) circuitry indicated in FIGS. 1 and 4.

FIG. 18 is a logic diagram of the ready (RDY) circuitry indicated in FIGS. 1 and 4.

FIG. 19 is a logic diagram of the M/X and mode select decoding circuitry indicated in FIGS. 1 and 4.

FIGS. 20A and 20B are diagrams illustrating the pin connections for packages in which the microprocessor chip of FIG. 1 can be encapsulated to provide a sixteen bit microprocessor capable of emulating a 6502 microprocessor and a pin compatible replacement for prior 6502 microprocessor and having advanced operating capability, respectively.

FIG. 21 is a flowchart useful in explaining operation of the microprocessor of FIG. 1 in its "16 bit mode" and also in its "8 bit mode" to emulate a 6502 microprocessor.

DESCRIPTION OF THE INVENTION

Before describing the chip topographical layout, the architecture and general operation will first be described with reference to FIG. 4. Microprocessor 100 outputs sixteen addresses, A0-A15. The address outputs are generally designated by reference numeral 101. Address bus lines A0-A7 are connected to low order address buffers 121. Address lines A8-A15 are stored in high order address buffers 120. A bus enable conductor 150 connected to data bus/bank address buffer circuit 151 is also connected to and enables address buffers 120 and 121. Both low order address buffer 121 and high order address buffer 120 are loaded by means of internal address bus 103. The address output buffers 120 and 121 are set to a high impedance output state in response to the bus enable (BE) signal 151.

Microprocessor 100 includes eight bidirectional data bus conductors D0-D7, generally designated by reference numeral 119. These eight conductors also can be used to output a eight bit "bank address" representing a third address byte. Accordingly, the signals and conductors designated by reference numeral 119 are also individually designated by DB/BA0-DB/BA7. .phi.2 is the main system clock, and since 24 address bits are always available for chip 100, address bits A0-A15 and bank address bits BA0-BA7 are output during the portion of the .phi.2 time period during which .phi.2 is low. When .phi.2 is high, data read/write operations can be performed on the output conductors 119.

A data bus enable (DBE) conductor 145 and a bus enable (BE) conductor 150 are connected between data bus and bank address buffer circuits 105 and buffer control circuitry 152. External data bus enable signals DBE and bus enable signal BE are applied to inputs to buffer control circuit 152, which are gated to conductors 145 and 150 in response to control signals on conductor 153 and external inputs BE or DBE.

The bus enable (BE) signal 150 allows external "three-stating" control of the data bus buffers 105 and the address buffers 120 and 121. For normal operation, the BE signal is high, causing the address buffers 120 and 121 to be "active" and also causing the data bus buffers to be "active" during a write cycle. When external control of the address lines 101 is desired, the BE signal is held low to disable the address output buffers 120, 121. The data bus enable (DBE) signal allows external control of the three-state data output buffers contained in block 105. To disable the data bus externally, the DBE signal should be held low.

The function of internal address bus 103 is simply to effectuate transfer of data from the internal registers to the address buffers 120 and 121. Internal bus address bus 103 is internally bidirectionally coupled to sixteen bit stack pointer register 111, sixteen bit arithmetic logic unit circuitry 110, sixteen bit program counter circuitry 108, and sixteen bit direct register 154. Sixteen bit index register 112 can be loaded onto internal address bus 103, as can sixteen bit data latch 106.

Data bus buffer circuitry 105 receives all of the basic information for the microprocessor 100 in the form of op codes, addresses and operands, one byte at a time. Data bus buffer 105 then supplies such information to the appropriate bytes of data latch 106, from which that information can be transferred to the appropriate bytes of internal address bus 103, internal data bus 141, pre-decode circuitry 126, or to instruction register 118. Pre-decode circuit 126 is actually a subsection of instruction register 118, and generates control signals that are applied to register transfer logic circuitry 131 and timing control circuitry 122 in response to the present instruction.

Program counter 16 actually includes four different groups of circuitry, including a low order program counter latch, a high order program counter latch, a low order program incrementer, and a high order program incrementer. The two program latches store the address of the present instruction until it is over, even during execution of an abort instruction. The generalized function of the program counter 108 is to provide sequential addresses for the program being executed.

Accumulator circuitry 109 includes a high order byte and a low order byte and contains the results of all data operations performed by the arithmetic logic unit circuitry 110.

Internal special bus 114 is a sixteen conductor bus which can be coupled by means of transfer switches 142 to sixteen bit internal data bus 141 in response to appropriate decoding transfer signals from register transfer logic 131. Internal special bus 114 is bidirectionally coupled to index register 113, index register 112, stack pointer register 111, arithmetic logic unit circuitry 110 and accumulator circuitry 109, all of which are sixteen bit circuits. Internal data bus 141 is bidirectionally coupled to sixteen bit accumulators 109, sixteen bit program counter 108, eight bit direct register 54, eight bit program bank register 155, eight bit data bank register 156, and eight bit data latch 106.

Arithmetic logic unit 110 includes four subsections, including a low order binary ALU, a low order decimal ALU, high order binary ALU, and a high order decimal ALU. Arithmetic logic unit 110 performs all the basic logic operations such as addition, substraction, multiplication, exclusive ORing logical ANDing, logical ORing, bit testing, etc., and can also compute addresses, which is why it has connections to both the internal address bus 103 and the internal bus 114 and the internal data bus 141.

Stack pointer register 111 keeps track of stack locations in memory and points to them. The two index registers 112 and 113 enable a software program to simultaneously point to two different software tables.

Reference numeral 117 designates a nine bit register referred to as the status register. It contains individual operating status flags for indicating the status of microprocessor 100. These flags include a negative (N) bit, an overflow (V) bit, a memory select (M) bit, an index register select (X) bit, a decimal mode (D) bit, an interrupt disable (I) bit, a zero (Z) bit, a carry (C) bit, and an emulation (E) bit. As subsequently explained in more detail, if the (E) bit is a "one", the microprocessor 100 emulates the well-known 6502 eight bit microprocessor. If the (E) bit is a "zero", the microprocessor 100 is said to be in its "native" mode and does not emulate the 6502. If the (E) bit is a "one", the microprocessor forces the (X) and (M) bits to also be "ones". If the (X) bit is a "one", it in effect masks off the upper byte of the X index register 112, allowing only eight-bit indexing operations, which are the type performed by the 6502 microprocessor, but which can also be performed by a sixteen bit microprocessor that might need to perform an eight bit indexing operation. If the (X) bit is a "zero", this causes index register 112 to perform a sixteen bit indexing operation. The M bit has the effect of "extending" the eight bit op codes in the instruction register. If the (M) bit is a "one", this has the effect of causing eight bit operations on the accumulator and on memory. If the M bit is a "zero" this has the effect of causing the same operations, i.e., sixteen bit operations, on all sixteen bits of the accumulator and causes all operations on external memories to be sixteen bit operations.

The microprocessor chip contains a simple "power up" circuit that, in addition to forcing certain other initial states in the microprocessor when it is initially powered on, sets the (E) bit at a "one". All microprocessors and most dynamic MOS logic circuits have such "power up" circuitry, which can be readily provided by those skilled in the art and therefore is not described in further detail. If E is a "zero", the microprocessor operates in the state determined by the X and M bits.

The basic timing for microprocessor 100 is contained by clock generator circuit 156. The addresses are output during the .phi.2 period when .phi.2 is low, or during .phi.2 "low time", or .phi.2L. During .phi.2 "high time" (.phi.2H) data is transferred on the external data bus conductors 119. 4 is utilized to establish internal timing waveform edges which minimize the amount of internal logic circuitry associated with addressing certain random access memories requiring a row address strobe (RAS) signal and a column address strobe (CAS) signal. The OSC*(OUT) signal is applied to a crystal whose other terminal is connected to the input of the inverter havings its input connected to the .phi..alpha.(IN)/-2(IN/CLK) conductor to provide enough gain to create oscillation using the external crystal. The .phi.1/(OUT) signal is the inverted .phi.2(OUT) signal to provide timing for external R/W* operations.

Note that an asterik (*) is used herein to denote the "zero" or "false" state of a logic variable, since the conventional "bar" is unavailable on the printer being used by applicant.

In block 122, timing control or timing generator circuitry produces sixteen timing states that are respectively stepped through in sequence. This circuitry is reset each time a new instruction is begun and is incremented in accordance with the number of cycles, from one to sixteen required for execution of that instruction. The outputs of timing control circuit 122 are connected to inputs of the instruction decode circuitry, and more specifically to the minterm decoding circuitry 116A of the instruction decode circuitry. These outputs, in conjunction with output signals from instruction register 118, are operated upon by the instruction decoder minterm circuitry 116A to generate intermediate signals needed to produce register transfer signals such as 131-1 to 131-11, which are produced by the register transfer logic circuitry 131. The register transfer logic simply latches the intermediate signals (subsequently described as sum-of-minterm signals or register transfer signals) and then clocks the latched register transfer signals onto conductors 131-1, etc., at the appropriate times to effectuate the desired information transfers.

The read/write* (R/W*) signal is normally in the high state, indicating that microprocessor 100 is reading data from memory or from an input/output bus. In the low state of the read/write* (R/W*) signal, the data bus has valid data to be output from the microprocessor to the address memory location specified by address lines A0-A5 and DB/BA0-DB/BA7 lines. The R/W* conductor can be set to a high impedance output state by the bus enable signal (BE) on conductor 150.

The system control circuitry 157 receives signals from the register transfer logic circuitry 131 and the E bit of the status register 117, and produces outputs on the R/W* conductor, the VPA (valid program address) the VDA (valid data address), the memory lock (ML)* signal, the vector pull (VP)* conductor, the emulate (E) conductor, and the M/X conductor. The system control circuitry in block 157 includes the subsequently described circuits shown in FIGS. 13-19.

The status register 117 is bidirectionally coupled to the register transfer logic 131.

An internal sync signal produced by timing control circuit 122 is provided to identify cycles of instruction execution during microprocessor operation. If the ready (RDY) line is pulled to a low state level during the .phi.2 low times, microprocessor 100 will stop in its current state and will remain in that state until the RDY line goes high, whereby the sync signal can be used to control the RDY signal so that it causes single instruction execution.

The memory lock (ML)* signal output by timing control circuit 122 indicates the ned to defer the rearbitration of the next bus cycle to ensure the integrity of read-modify-write instructions. The (ML)* signal goes low during the ASL, DEC, INC, LSR, ROL, ROR, TRB, and TSB memory referencing instructions, which are well-known to those in the art familiar with programming of the 6502 microprocessor.

Interrupt logic circuitry contained in block 115 has outputs bidirectionally connected to the request transfer logic 131. The interrupt request (IRQ) signal requests an interrupt signal to be executed by microprocessor 100. If the interrupt flag in the status register 117 is a "zero", the current instruction is completed and the interrupt sequence begins during .phi.2 low time. The program counter and status register contents are stored in a "stack" in external memory. The microprocessor will then set the interrupt mask flag high so that no further interrupts may occur. At the end of the cycle, the low order program counter register 108 will be loaded from the hexadecimal address OOFFFE in the emulation mode and OOFFEE in the native mode, and the program counter will be located from the hexadecimal location OOFFFF in the emulation mode, and OOFFEF in the native mode, thereby transferring program control to the memory vector located at these addresses. The RDY signal must be in the high state for any interrupt to be recognized.

The non-maskable interrupt (NMI)* input of interrupt logic 115 makes interrupt requests in response to a negative-going edge to the effect that a non-maskable interrupt sequence is to be generated within microprocessor 100. The current instruction is completed and the interrupt sequence begins during the following .phi.2 low time. The program counter is loaded with the interrupt vector from the locations OOFFFA for the emulation mode and OOFFEA for the native mode for the low order byte and the locations OOFFFB for the emulation mode and OOFFEB for the native mode for the high order byte, thereby transferring program control to the non-maskable interrupt routine.

The reset (RES)* input to interrupt logic 115 causes an initialization sequence to begin by means of a positive transition from a (RES)* input signal. The reset signal must be held low for at least two clock cycles after the power supply voltage V.sub.DD reaches its operating voltage from a powered down condition, after which time R/W* is high and sync is low. When a positive edge is detected on the (RES)* line, initialization sequence lasting six clock cycles occurs. The interrupt mask flag is set, the decimal mode bit of the status register 117 is cleared, and the program counter 108 is loaded with the reset vector from the locations OOFFFC for the low order byte and OOFFFD for the high order byte which is the start location for program control.

The decoding of the present instruction by means of the minterm instruction decoding circuitry 116A and sum-of-minterm section 116B produces signals which drive the register transfer logic circuitry 131. The output signals thereof 131-1 to 131-11 are coupled to transfer of the various registers in the "register section" of FIGS. 1 and 4 and generate the necessary enable signals to effectuate transfer of data between the various busses and various registers and the arithmetic logic unit 110. All control for transferring data between the various registers, the arithmetic logic unit, and the various busses is accomplished by these and other such transfer signals produced by decoding of the instruction op codes.

The first level instruction decoding in minterm circuitry 116A generates 498 minterm signals, as opposed to 252 for the 6502 microprocessor described in the above-referenced parent application. The second level of decoding in block 116B produces 132 "sum-of-minterm" signals which are used to set or reset clocked latches contained in the register transfer logic circuitry 131. At appropriate times the states of these clocked latches are output in response to appropriate clock signals to produce the register transfer signals to actually effectuate the various data transfers between the registers, arithmetic logic unit, and internal busses, required for execution of the present instruction.

Four entirely new signals are included in the sixteen bit microprocessor, when operating in its "native" mode, i.e., with the emulate (E) bit equal to "zero". These include an (ABORT)* input which can interrupt the currently executing instruction without modifying internal registers. The valid data address (VDA) and valid program address (VPA) outputs facilitate dual cache memory by indicating whether a data segment (fast memory) or program segment (slow memory) is accessed. Modifying or prioritizing a vector is made easy by monitoring the vector pull (VP)* output.

The ABORT input is used to abort other instructions presently being executed as a result of an error condition, such as an incorrect address appearing on the address bus. FIG. 14, subsequently described, discloses the specific abort circuitry. A negative-going edge of the ABORT input "aborts" the present instruction from the cycle during which the abort signal is pulled to a low level. This prevents any and all internal registers accessible by the programmer from being modified. At the end of the present instruction, an "interrupt-like" operation pushes the program counter, program status register and bank register (in the native mode only) onto the external memory stack, sets the interrupt status register flag to a "one" sets the decimal flag to "zero", sets the program bank register to "zero", and loads the program counter with the contents of address locations OOFFF8 and OOFFF9 for the emulation mode and OOFFE8 and OOFFE9 in the native mode. The ABORT signal cannot be masked, and specifically prevents internal transfers into or out of the data bank registers, the program bank register, the status register, the direct register, the accumulator registers, the stack registers, and the X and Y index registers until the abort logic is reset. However, the abort input does not affect any increments or the arithmetic logic unit circuitry. In order to implement the desired operation of the abort logic, none of the foregoing registers are ever updated until after an instruction is completed. Therefore, if the negative-going edge of the ABORT input occurs during the execution of a particular instruction, and if the execution of that instruction is not complete, the contents of the foregoing registers remain intact, since any transfers are inhibited by the abort circuitry. The interrupt-like operation causes the microprocessor 100 to go in a conventional manner to a vector address of an abort subroutine. After the abort subroutine has been completed, the microprocessor can return and readily re-execute the same instruction that was being executed when the abort occurred because none of the internal registers have changed.

The vector pull (VP)* output goes low during the two cycles when a vector address is being pulled, for example in response to an (IRQ)* active input. The vector pull output goes low for all interrupt vector pulls, and for the (ABORT)*, BRK, COP, (IRQ)*, (NMI)*, and (RES)* signals. The vector pull output thus may be used to modify and/or prioritize interrupt routines. The vector pull logic is shown in FIG. 15. The valid data address (VDA) and valid program address (VPA) outputs indicates the type of memory being addressed by the address bus. If both VDA and VPA are "zeros", the present microprocessor operation is entirely "internal" and the address and data busses are both available. If VDA is "zero" and VPA is a "one", a valid "program" address is on the address bus 101. (Program addresses are usually for a relatively slow memory, such as read-only memory or floppy discs.) This signals other circuitry, such as a memory controller that the clock signal can be operated at a slow rate. If VDA is a "one" and VPA is a "zero", this indicates to an external memory controller that there is a valid data address on the bus, and rapid access may be desired. If both VDA and VPA are "ones", an operational code (op code) fetch operation is occurring. The VDA and VPA outputs may be used for virtual and cache memory control.

Those skilled in the art can obtain various designs for the various latching, arithmetic, and other logic circuits shown in FIG. 4. All of these types of circuits have been implemented in N-channel MOS in the widely used 6502 microprocessor; CMOS versions can be implemented in various ways by those skilled in the art, and a coupled circuit schematic of the chip 100 is not needed to enable one skilled in the art to produce the invention.

Referring now to FIG. 1, the topography of microprocessor chip 100 will be described, indicating the locations of the various major blocks of circuitry on the chip. Chip 100 includes a flat surface with a top edge 11, a left edge 12, a bottom edge 13, and a right edge 14. The vertical dimension of chip 100 is 278 mils, and its horizontal dimension is 164 mils. (Note that the same reference numerals, with subscripts if appropriate, are used in FIG. 1 to indicate the same or corresponding blocks of circuitry in FIGS. 1 and 4.) The low order address buffer circuitry 121 is 2052 microns in the vertical direction and 716 microns in the horizontal direction, and is positioned along the lower left-hand portion of edge 12 and the lower right-hand portion of edge 13 (There are 25.4 microns per mil.) High order address buffers 120 are 566 microns in the vertical direction and 5436 microns in the horizontal direction, and lie along bottom edge 13 of microprocessor chip 100. Data bus buffers 105 are 2030 microns in the vertical direction and 906 microns in the horizontal direction, and extend from bottom edge 13 upward along right edge 14. The "register section" of microprocessor 100 is surrounded on three sides by low order address buffers 121, high order address buffers 120, and data bus buffers 105. The register section is 5436 microns in the horizontal direction and 1136 microns in the vertical direction and includes the following blocks of circuitry, the dimensions of which are given below in Table 1. The vertical dimension of each of the blocks in the register section is 1136 microns.

TABLE 1______________________________________ WIDTH INNAME MICRONS______________________________________Low order address latch 121A 296X register 112 120Y register 113 120Low order stack register 111A 110High order stack register 111B 126Bus transfer and precharge 138circuitry 158Low order binary ALU 110A 486Low order decimal ALU 110B 446High order binary ALU 110C 538High order decimal ALU 110D 446Low order accumulator 109A 138High order accumulator 109B 120Low order program incrementer 107A 236Low order program counter latch 107B 61High order program counter latch 107C 61High order program incrementer 107D 232Direct register 154 176Precharge circuitry 160 88High order address latch 120A 72Status register 117 488Program bank register 155 88Data bank register 156 and 196Input bank incrementer andtransfer logic 156AHigh order output data latch 106A 114Low order output data latch 106B 110Temporary register and incrementer 159 256High order input data latch 106C and 174low order input data latch 106D______________________________________

As is known to those skilled in the art, precharge circuitry such as that included in blocks 158 and 160 is required in all dynamic MOS circuitry to set initial values of certain conductors in response to certain precharge signals. For example, such precharge circuitry is used to preset the data bus conductors and address bus conductor lines to initial values at the beginnings of various operation cycles and to set certain bits in certain registers and latches to specified "one" or "zero" initial levels.

Non-maskable interrupt (NMI) logic circuitry, which is 530 microns by 628 microns, is located in the upper left corner of chip 100. (Hereinafter, when dimensions of blocks of circuitry are given, the horizontal dimension will be given first and then the vertical dimension.) Memory lock logic 168 (560 by 628 microns) is located immediately to the right of NMI logic 115A. IRQ logic 115B (444 microns by 628 microns), abort logic 167 (480 microns by 628 microns), ready I/O logic 166 (426 microns by 628 microns), vector pull logic 165 (356 microns by 628 microns), reset interrupt logic 115C (714 microns by 628 microns), VDA logic 164 (460 microns by 628 microns), status register and M/X output logic 117A (546 microns by 628 microns), clock generator oscillator circuitry 156 (896 microns by 628 microns), and DBE and BE logic circuitry 163 (538 microns by 628 microns) are positioned from left to right along the top edge 11 of chip 100. Emulate bit output is located in the extreme upper right-hand corner of chip 100, and R/W* is located immediately below emulation bit output logic 161.

Instruction register 118 (840 microns wide) is located immediately below R/W* logic 162, and predecode circuit 126 (525 microns by 525 microns) is located between instruction register 118 and data bus buffer circuitry 105. Sixteen state timing control circuitry 122 is located along the left edge 12 of chip 100, as indicated in FIG. 1, occupying 780 microns by 744 microns, VDA logic 157A (340 microns by 570 microns) is located in the lower left corner of block 122.

It should be noted that many (41) bonding pads which are located around the periphery of microprocessor chip 100 are not shown in FIG. 1. These bonding pads are included in the various blocks or areas, such as data bus buffers 105, higher address buffers 120, data bus buffers 105, etc. to effectuate wire bonding of the input and output conductors of chip 100 to leads of a package such as the one in FIG. 20A. FIGS. 20A and 20B include diagrams of two pin configurations, one for the sixteen bit operational version of the microprocessor 100, and the other for the circuit when it is set up to be pin compatible with the prior 6502 microprocessor.

Immediately above the register section is a N-channel sum-of-minterms section 116B, which is approximately 752 microns in the vertical direction. Mode select decode circuitry 160 (866 microns by 210 microns), occupies the upper left-hand corner of block 116B, however.

Above the sum-of-minterm section 116B is a block 116C (198 microns in the vertical direction), containing 498 CMOS inverters, which invert 498 N-channel minterms contained in block 116A, which is 346 microns in the vertical direction.

A conductor 169 from abort logic 167 is routed around blocks 116A, 116B and 116C, and is connected to inhibit data transfers in the X and Y index registers 112 and 113, the high and low order stack registers lllA and lllB, the high and low order accumulaters 109A and 109B, the direct register 154, the status register 117, the program bank register 155, the low order program counter latch 107B, the high order program counter latch 107C, and the data bank register 156. An emulate conductor connected to the E bit in status register 117 is connected to force higher byte to be a "zero" in each of the X and Y index registers 112 and 113, and forces a hex 01 in the high order stack register lllB, and is also routed to the E output logic block 161 to produce an output signal indicative of whether the microprocessor 100 is emulating a 6502 microprocessor or is operating in its "native" sixteen bit mode.

The scale images of the photomasks shown in Figs. 6-11, 12A, and 12B are included as being of interest in that they show the density of circuitry and other features of interest to microprocessor chip designers. It would be possible to determine the precise layout of the microprocessor chip of the present invention from the information contained in the photomasks, although one skilled in the art ordinarily would not resort to this expedient because it would be more practical to follow the teachings made with reference to the other figures herein and, in accordance with the various permissible line spacings and line widths for a particular CMOS manufacturing process, use various available CMOS latch circuits, inverters, and arithmetic circuits that he prefers. The teachings herein with reference to the instruction decoding circuitry, in sections 116A, 116B, and 116C, would have to be followed in order to obtain the benefits of the invention.

The main sections or blocks indicated in FIG. 1 are superimposed in heavy lines on the scale negative image metalization pattern of FIG. 5.

In accordance with the present invention, the improvement of the layout of the instruction decode portion of the chip, including minterm read-only memory decoding circuitry 116A, minterm inverter drivers 116C, and sum-of-minterm read-only decoding circuitry 116B made it possible to reduce the width of the instruction decode section of the chip 100 by approximately 30% over what would have been required if only the techniques used in the layout of the instruction decode section of the above-referenced parent application were used. The layout techniques of the present invention also reduced the "vertical" dimension of the instruction decode region, including all of the necessary wire routing of register transfer signals to the register section of approximately 40%.

If I had not been able to achieve this reduction in the instruction decode circuitry of the chip, its width would have been roughly 65 mils greater and its heighth would have been about 16 mils greater.

FIG. 3C of the parent application illustrates the basic layout of N-channel minterm circuitry 116A and sum-of-minterm circuitry 116B of the microprocessor chip 100. FIG. 3B of the present application more accurately represents the layout structure of N-channel minterm circuitry 116A of the present application, wherein the diffused minterm lines 175, and hence all the N-channel MOSFETs formed therein, are each nine microns wide and are spaced apart by two microns. The horizontal polycrystalline silicon lines 176 are 5 microns wide. Metal jumpers such as 177 are utilized between aligned portions of the diffused lines 175 where no N-channel FET gate is desired. Thus, the average center-to center spacing in the horizontal direction between successive vertical diffused minterm lines is eleven microns. The nine micron width of the gate regions such as 178 is conventional for dynamic N-channel MOS NAND gates in this type of circuit, and is necessary to provide adequate output node discharge times in a read-only memory array such as 116A. When I designed this circuit for the microprocessor in the above-referenced parent application. I believed, after performing a computerized circuit analysis, that the MOS gates 178 had to be at least nine microns wide.

Even so, the instruction decoding in the microprocessor of the parent application was not as fast as desired. One reason for this is that I encountered a difficult design trade-off involving the capacitance of the horizontal polycrystalline silicon conductors 176 from the instruction register and the accumulated capacitance of the various MOS gates such as 178 in the N-channel minterm circuitry 116A. Making the MOSFET gates wider decreased the minterm signal propagation time in vertical direction, but increased the horizontal signal propagation along the polycrystalline silicon lines 176 by increasing the capacitance and the resistance of the polycrystalline silicon lines 76, and hence increasing their RC time constant.

At that time, I believed that in order to design a sixteen bit CMOS microprocessor which would be suitable as a "follow-up" product to the CMOS microprocessor of the present application, many more diffused minterm lines would be required, and it appeared to me that the minterm circuitry 116A therefore would be much wider and slower than desired. But I did not see any solution to this problem.

Returning to the present invention, the basic layout in minterm circuitry 116A will be described and then, after also describing the layout in the sum-of-minterm circuitry 116B of FIG. 2, the combined decoding operation of the two sections will be explained, indicating the resulting improvement in operating speed and savings in chip area.

In FIG. 3A, reference numerals 175, with appropriate letters, are used to designate the diffused minterm lines. Each of the diffused lines 175 is only five microns wide, rather than nine microns as in FIG. 3B. This almost doubles the channel resistance of the N-channel MOSFETS formed, slowing down signal propagation downward through the minterm region 116A. Reference numeral 179 designates horizontal metal lines (rather than the horizontal polycrystalline silicon lines 176 in FIG. 3B) which are connected to the outputs of the instruction register. These metal lines have far lower resistance than the polycrystalline silicon lines, so the RC time constant of lines 179 is much less than that of the polycrystalline silicon conductors 176 of FIG. 3B. The polycrystalline silicon gates required for each of the N-channel MOS transistors such as 178 in FIG. 3A make electrical contact to the metal lines 179 by means of contacts openings 180. Since the accumulated gate capacitance associated with each of the metal lines 179 has been nearly halved, the output drive current needed from the instruction register latches is nearly halved. This, coupled with the greatly reduced time constant associated with each of the line 179 greatly increases the signal propagation time along the metal lines 179.

Referring next to FIG. 2, reference numeral 116C discloses a plurality of CMOS minterm inverter drivers, each having its input connected to the bottom of a respective one of the minterm lines 175. Inverter drivers 116C, although they have a relatively high gate capacitance that further slows signal propagation downward through the N-channel minterm region 116A, are capable of driving the capacitance associated with the polycrystalline silicon conductors 181, 181A, 181B, etc., more effectively than the minterm lines and minterm MOSFETs 178 of FIG. 3B could if the inverter drivers 116C were to be omitted. Furthermore, the signal inversion produced by the inverter drivers 116C makes it possible to obtain logical NORing in sum-of-minterm circuitry 116B using N-channel, rather than P-channel MOSFETs that were used in section 116B of FIG. 3C of the parent application. (Such N-channel MOSFETs have only about one-half of the channel resistance of P-channel MOSFETs of the same geometries, making it possible for there to be considerably faster downward signal propagation of the inverted minterm signals through sum-of-minterm region 116B than previously.)

In FIG. 2, reference numerals 182, with appropriate suffix letters such as A, B, etc., designate diffused regions that are attached to a V.sub.SS conductor 183 and extend vertically downward through circuitry 116B. The vertical diffused regions 182 have horizontal extensions such as 184 wherever an N-channel sum-of-minterm decoding MOSFET is required. The gate electrode of such N-channel sum-of-minterm MOSFETs are indicated by X's wherever such horizontal extensions 184 are crossed by vertical inverted minterm polycrystalline silicon conductors such as 185.

The drain electrodes of each of the sum-of-minterm MOSFETs are connected to horizontal metal conductors such as 186 by means of contact openings such as 187, thereby performing the minterm summing operation.

Each of the vertical polycrystalline silicon conductors 185 is connected to the output of a respective one of the minterm inverter drivers 116C.

The general approach of providing vertical diffused lines 182, horizontal diffused extensions 184, vertical polycrystalline silicon conductors 185, horizontal metal conductors 186, and contacts 187 is basically the same as in FIG. 3C of the parent application. However, using this technique (as I did on the chip of the parent application) on the chip 100 of the present invention would have resulted in a much wider sum-of-mintern region 116B than I actually achieved because of the much higher number of sum-of-minterm transfer signals that are required for the sixteen bit microprocessor chip of the present invention Furthermore, I was able to arrive at a reasonably optimum layout of the sum-of-minterm region in the parent application only after three very laborious trial layouts in which I followed the approach described in the above-referenced parent application to positioning the minterm summing horizontal metal lines 186 in such a way as to provide gaps through which diffused and/or polycrystalline silicon conductors could be "dropped" down into the register transfer logic where the resulting sum-of-minterm transfer signals were needed, to avoid utilization of an excessive amount of chip surface area for routing such signals.

However, the method of arranging the relative positions of the horizontal metal sum-of-minterm lines 186 is different than in the above-referenced parent application (Of course, the relative placement of the horizontal metal lines 186 determines the placement of all N-channel MOSFETs connected to a particular horizontal line, and hence determines how far down into sum-of-minterm region 116B the vertical poly (polycrystalline silicon) lines 185 and the vertical diffused lines 182 extend.)

In view of the fact that the number of minterms (498) is nearly double that in the above-referenced parent application, and in view of the great amount of time that was required by me to use the approach described in the parent application to arrange the positions of the horizontal metal lines so as to create "gaps" between diffused regions adequate for routing diffused or polycrystalline silicon register transfer signal lines more or less directly downward to the location in the register transfer section in which those lines were needed, I decided, after much reflection, to take an entirely differently, almost opposite approach to that in the present application, partly just to get at least part of the layout done and in the hope that at that point a better solution would occur to me. I therefore brought the vertical diffused lines 182 and the adjacent vertical polycrystalline silicon lines 185 which were to be included in those sum-of-minterm signals comprised of the fewest number of minterms, for example, only one or two minterms, all the way down to the lowest horizontal metal lines 186.

For example, in FIG. 2, assume that horizontal metal conductors 186-1, 186-2, 186 3 and 186-4 are the sum-of-minterm conductors having the fewest N-channel MOSFETs (denoted by X's), i.e., minterms, connected thereto More specifically, sum-of-minterm conductors 186-1, 186-2 and 186-3 each have only one or two minterms. Only one minterm actually is shown for each of these lines in FIG. 2, but assume that another located to the left or right of the section shown in FIG. 2 might also be connected. Sum-of-minterm metal conductor 186-4 has two minterms connected thereto, and possibly another to the right or left to the section shown in FIG. 2. I continued this pattern, working my way from the bottom horizontal sum-of-minterm conductor up, and each time I connected all the minterms, (only a few) to the next highest horizontal sum-of-minterm conductor 186, I also brought downward a vertical diffused or polycrystalline silicon conductor such as 188 downward, using "cross-unders" of a different type where necessary, to the register transfer gates. The point at which each of the vertical lines 188 was connected to the various sum-of-minterm horizontal metal sum-of-minterm lines 186 was located as directly as possible above the point in the register section at which that transfer signal, i.e., the sum-of-minterm transfer signal, was needed. By the time I had done this operation for all of the sum-of-minterm lines comprising less than about approximately four or five minterms, nearly 30% of the sum-of-minterm gate transfer signals had been routed to their proper destination, leaving only 77 to go, all of which, of course, included more than five or six minterms, and which would be much more difficult to route than the first group.

I then decided that the most difficult-to-route sum-of-minterm signals, i.e., those comprising the largest numbers of minterms, should be located at the top of the sum-of-minterm array, and that these metal lines should be generally routed around the right-hand side of the sum-of-minterm array, as indicated by reference numeral 189 in FIG. 2, and brought back to the right, as indicated by reference numeral 189A, to locations directly above where the particular sum-of-minterm transfer signals would be needed in the register section of the chip 100, using cross-unders as necessary. I thus began working my way from the top sum-of-minterm metal conductors 186 downward for the sum-of-minterm lines having large numbers of minterms, i.e., N-channel MOSFETs, connected thereto. It was my hope that by positioning the horizontal metal sum-of-minterm conductors containing the fewest numbers of minterms at the bottom and those containing the greatest numbers of minterms at the top of the sum-of-minterm array, there would be enough gaps, i.e., regions with adequate spacing between diffused regions, in the intermediate region to allow fairly direct downward routing of diffused and/or polycrystalline silicon conductors from the sum-of-minterm conductors containing intermediate numbers of minterms, located more or less directly above where those intermediate sum-of-minterm transfer signals were required in the register section.

As this approach to layout of the sum-of-minterm region 116B progressed, my foregoing hunch turned out to be correct, and with much less effort than I thought might have been required, I was able to complete the layout of the sum-of-minterm region with much less expenditure of time, and with only approximately 40% less chip area than I thought would have been required if I had not hit upon this approach.

Note that in FIG. 2, reference arrows 190 simply designate continuations of the diffused or polycrystalline silicon sum-of-minterm transfer lines 188 more or less directly downward into the register transfer section.

FIG. 2A schematically discloses the structure of both the minterm region 116A and the sum-of-minterm region 116B and also the minterm inverter drivers 116C described above with reference to FIGS. 2 and 3A, and further discloses the mode select decode circuitry 160, in which the M and X bits of the status register 117 are included in the decoding function performed by the sum-of-minterm decoding section 116B in order to produce the transfer signals CYL/CF (low order carry bit to carry flag), CYH/CF (high order carry bit to carry flag), DBIL/N (data bus low bit 7 to the N flag), DBIH/N (data bus high bit 7 to the N flag), and also the transfer signals "output latch low" to DBO and "output latch high to DBO", and also "special bus low to accumulator A" and "special bus high to accumulator B", and "zero detect of the low byte to the Z flag" and "zero detect low and high to the Z flag", "reset zero bit of the timing generator", and "set timing generator bit 1".

Table 2 contains a list of all of the sum-of-minterm transfer signals for chip 100 and their functional descriptions in abbreviated language that one skilled in the art familiar with the diagram of FIG. 4 will readily understand.

TABLE 2______________________________________Sum-of-MintermNumber Symbol Description______________________________________ 1. A/DBL Transfer A accumulator to data bus low 2. A/SBL Transfer A accumulator to special bus low 3. AB/AL Transfer address bus to address latch 4. AB/PI Transfer address bus to program incrementer 5. ABH/AXH Transfer address bus high to ALU X input high 6. ABL/AXL Transfer address bus low to ALU X input low 7. ADD Add decimal 8. AL/AB Transfer address latch to address bus 9. AUH/ABH Transfer ALU high to address bus high 10. AUH/DBH Transfer ALU high to data bus high 11. AUH/SBH Transfer ALU high to special bus high 12. AUL/ABL Transfer ALU low to address bus low 13. AUL/DBL Transfer ALU low to data bus low 14. AUL/SBL Transfer ALU low to special bus low 15. B/DBH Transfer B accumulator to data bus high 16. B/SBH Transfer B accumulator to special bus high 17. BRKCOPV Execute BRK or COP instruction 18. BRKE Executing BRK in emulation mode 19. BRNCHE Executing branch instruction in emulation mode 20. CF/A7H Transfer carry flag to ALU high bit 7 21. CF/A7L Transfer carry flag to ALU low bit 7 22. CF/CIL Transfer carry flag to ALU low carry in 23. CLRV Clear V flag 24. COPV Force COP vector 25. CYH-BKI Transfer ALU carry high compliment to bank incrementor input 26. CYH/BKI Transfer ALU carry high to bank incrementor input 27. CYH/CF Transfer ALU carry high to carry flag 28. CYL/CF Transfer ALU carry low to carry flag 29. D/DB Transfer direct register to data bus 30. DB/D Transfer data bus to direct register 31. DB/PI Transfer data bus to program incrementor 32. DB/SB Transfer data bus to special bus 33. DBH-/AXH Transfer data bus high compliment to ALU X input high 34. DBH/AXH Transfer data bus high to ALU X input high 35. DBH/N Transfer data bus high bit 7 to N flag 36. DBH/NEG Transfer data bus high bit 7 to NEG latch 37. DBH/OLH Transfer data bus high to output latch high 38. DBH/V Transfer data bus high bit 6 to V flag 39. DBI/ILH Transfer input data bus to input latch high 40. DBI/ILL Transfer input data bus to input latch low 41. DBI/T Transfer input data bus to temporary register 42. DBL-AXL Transfer data bus low compliment to ALU X input low 43. DBL/AXL Transfer data bus low to ALU X input low 44. DBL/CF Transfer data bus low bit 0 to carry flag 45. DBL/DBH Transfer data bus low to data bus high 46. DBL/DBR Transfer data bus low to data bank register 47. DBL/N Transfer data bus low bit 7 to N flag 48 DBL/NEG Transfer data bus low bit 7 to NEG latch 49. DBL/OLL Transfer data bus low to output latch low 50. DBL/P Transfer data bus low to status register 51. DBL/PBR Transfer data bus low to program bank register 52. DBL/V Transfer data bus low bit 6 to V flag 53. DBR/DBL Transfer data bank register to data bus low 54. DBR/DBO Transfer data bank register to output data bus 55. DCE Decimal carry enable 56. DH/ABH Transfer direct register high to address bus high 57. DL/ABL Transfer direct register low to address bus low 58. EOR Execute EOR operation in ALU 59. GOTO6 Force timing generator to state 6 60. GOTO7 Force timing generator to state 7 61. GOTOZ Force timing generator to state 0 62. HLDOLD Hold previous address in address latches 63. ILH/ABH Transfer input latch high to address bus high 64. ILH/DBH Transfer input latch high to data bus high 65. ILL/ABL Transfer input latch low to address bus low 66. ILL/DBL Transfer input latch low to data bus low 67. IR5 Instruction register bit 5 68. IR5/CF Transfer instruction register bit 5 to carry flag 69. IR5/D Transfer instruction register bit 5 to decimal flag 70. IR5/I Transfer instruction register bit 5 to interrupt mask flag 71. LIR Load instruction register 72. ML Enable memory lock output 73. NEG/AYH Transfer NEG latch to ALU Y input high 74. 01/CIH Force 1 to ALU carry in high 75. 01/CIL Force 1 to ALU carry in low 76. OLH/DBO Transfer output latch high to output data bus 77. OLL/DBO Transfer output latch low to output data bus 78. OR Force OR operation in ALU 79. P/DBL Transfer processor status register to data bus low 80. PBR/DBL Transfer program bank register to data bus low 81. PBR/DBO Transfer program bank register to output data bus 82. PC/AB Transfer program counter register to address bus 83. PC/DB Transfer program counter register to data bus 84. PI/AB Transfer program incrementor to address bus 85. PI/DB Transfer program incrementor to data bus 86. PICIN Increment program incrementor 87. R0 Reset timing generator bit 0 88 Rl Reset timing generator bit 1 89. R2 Reset timing generator bit 2 90. RABORTl Reset abort latch 91. S/AB Transfer stack register to address bus 92. S/SB Transfer stack register to special bus 93. S0 Set timing generator bit 0 94. Sl Set timing generator bit 1 95. S2 Set timing generator bit 2 96. S3 Set timing generator bit 3 97. SB/AB Transfer special bus to address bus 98. SB/S Transfer special bus to stack register 99. SB/X Transfer special bus to X register100. SB/Y Transfer special bus to Y register101. SBH/AYH Transfer special bus high to ALU Y input high102. SBH/B Transfer special bus high to B accumulator103. SBL/A Transfer special bus low to A accumulator104. SBL/AYL Transfer special bus low to ALU Y input low105. SDD Substract decimal106. SR Force shift right operation in ALU107. SR8/A7L Transfer ALU bit 8 to ALU bit 7108. STP Stop the clock109. SUM Force sum operation in ALU110. T/DBO/1 Transfer temporary register to output data bus during phase 2 low time111. T/DBO/2 Transfer temporary register to output data bus during phase 2 high time112. TSTL/Z Transfer test low byte to zero flag113. TSTLH/Z Transfer test word to zero flag114. V/ADL Transfer vector to address bus low115. VDA Enable valid data address output116. VH/V Transfer ALU overflow high to V flag117. VL/V Transfer ALU overflow low to V flag118. VPA Enable valid program address output119. WAI Force wait operation120. WRITE Enable write operation121. X/AB Transfer X register to address bus122. X/SB Transfer X register to special bus123. XCE Exchange carry and emulation bits in status register124. Y/SB Transfer Y register to special bus125. Z0/ADL0 Force 0 on address bus bit 0126. Z0/AYH Force zeros on ALU Y input high127. Z0/AYL Force zeros on ALU Y input low128. Z0/CIH Force zero into ALU carry in high129. Z0/DB Force zeros on data bus130. Z0/DBO Force zeros on output data bus131. ZL/Z Transfer byte zero detect output to Z flag132. ZLH/Z Transfer word zero detect output to Z flag______________________________________

In Table 2, the term "data bus low" refers to the low order byte on internal data bus 141, and similarly the term "data bus high" refers to the high data byte thereon. The term "special bus low" refers to the low order byte on internal special bus 114, and similarly for the term "special bus high". The same convention applies to the terms "address bus high" and "address bus low" with reference to internal address bus 103. The arithmetic logic unit circuitry of the chip 100 includes the high and low order binary and decimal arithmetic logic circuitry indicated in FIG. 1. Each such arithmetic logic circuitry unit includes an X input and a Y input which is referred to in various of the minterm descriptions in Table 2. The "input data bus" referred to in Table 2 refers to the bus going from the data bus/bank address buffer 105 to the sixteen bit data latch 106. The "output data bus" referred to in Table 2 refers to the eight bit bus going from program bank register 155, data bank register 156 and data latch circuit 106 to data bus/bank address buffer circuitry 105.

Next, important new functional sections in the sixteen bit microprocessor chip 100 which are not included in the microprocessor described in the above-referenced parent application are described with reference to FIGS. 13 through 19, including the emulation bit circuitry, the abort circuitry, the valid program address circuitry, the valid program address circuitry, the valid data address circuitry, the vector pull circuitry, improvements to the ready circuitry, and the M/X output circuitry.

Referring now to FIG. 13, reference numeral 161 designates the emulation bit output logic indicated in FIG. 1 The signal E.sub.OUT is an output signal that simply indicates whether microprocessor chip 100 is presently emulating the 6502 microprocessor, enabling external circuitry such as a memory controller or the like to readily determine how memory should be accessed. As in FIG. 1, reference numeral 155 designates the program status register. The C or carry bit, the X bit, the M bit, and the emulation or E bit are designated by reference numerals 155C, 155X, 155M and 155E, respectively. These bits are all connected to a conductor 170 that in turn is connected to an E bit 178A of instruction register 178 (FIGS. 1 and 4) which generates the E and E* lines that extend through the N-channel minterm decode circuitry 116A. Conductor 170 is also connected to X register 112, Y register 113, low order stack register 111A, high order stack register lllB, and to the input of the emulation output circuitry 161. The function of the emulation signal on conductor 170, if E is a "one", i.e., if the chip is emulating a 6502 microprocessor, is to produce a "one" on conductor 170 that causes E.sub.OUT to be a logical "one", forces a logical "one" into the X and M bits 155X and 155M, respectively, of the status register 155, and in effect only allows transfers out of the low order bytes of registers 112, 113, lllA, and lllB, and causes certain minterm decoding operations in circuitry 116A which are particular to the emulation mode of operation of chip 100. More particularly, if the E bit is a "one", this causes "zeros" to be forced into the high byte of each of registers 112 and 113 and forces a hexadecimal zero one into high order stack byte lllB.

The blocks 191 and emulation output logic 161 simply designate metal options on the metal layer of FIG. 12 which disconnect inverter 192 if chip 100 is packaged in the package of FIG. 20B (which is pin compatible with the 6502 microprocessor) and cause pin 35 of the package to be disconnected, as indicated by "N/C" in FIG. 20B.

Referring next to FIG. 14, the abort logic 167 of FIG. 1 is shown in detail. The ABORT signal is an input signal that is generated in response to external circuitry that detects an abort condition, for example an incorrect address on the address lines 101 (FIG. 4). The abort input conductor 193 is connected to a string of three inverters 194 and a clocked latch circuit 195 which can be reset by either the reset signal on pin 40 (FIG. 20A) or an internal circuitry RABORT (reset abort) signal which is generated when execution of an abort subroutine is completed.

The P-channel MOSFET designated by reference numeral 196 is a typical "pull up" resistance that maintains conductor 193 at a high level unless the negative-going ABORT signal is applied thereto. Clock latch 195 is clocked by a signal .phi.2I, an internal clock signal derived from the clock generator circuitry 156 in FIGS. 1 and 4 that gates or synchronizes all input gate transfers in the chip 100. Other internally generated clock signals referred to in FIGS. 14-19 include .phi.2O, which synchronizes all transfers from outputs of latches in the chip 100, .phi.2D, a delayed replica of .phi.2. .phi.4 is an internal transfer clock signal which generally performs the function of internal transfer timing for registers. .phi.4D is a slightly delayed version of .phi.4 which is required to precharge the instruction decode logic.

The output of clocked latch 195 is inverted and applied to a latch circuit 197 to generate a signal (ABORT..phi.1* ) which causes abort vector-generating circuitry 198 (which can be readily implemented by those skilled in the art) to force a vector address on address bus 101 (FIG. 4) representing the location of an abort subroutine that must be executed next. (.phi.1 is simply an alternate way of describing .phi.2 low time.)

The signal on conductor 199 is clocked into circuitry 197 by the clock signal .phi.2I. Herein, the symbols such as 200 designate CMOS transfer gates, which are well-known to those skilled in the art and simply constitute a P-channel MOSFET and an N-channel MOSFET having common sources and common drains. The small circle 201 on each CMOS transfer gate designates the gate electrode of the P-channel MOSFET. The opposite input designates the gate electrode of the N-channel MOSFET.

The (ABORT)* signal on conductor 199 is clocked into latch circuitry 202 by a pair of CMOS transfer gates of the type described above in response to a signal LIR (load instruction register) and is further synchronized from the output of the latch 202 into another latch 203 by another pair of CMOS transfer gates and in synchronization with the signal .phi.4. The output of the latch 203 is the (ABORT)* signal on conductor 169, which is routed to registers 107, 112, 113, lllA, lllB, 109A, 109B, 160, 117, 155, and 156 as previously described to prevent any transfers of data into those registers until the abort subroutine has been executed and the reset abort (RABORT) signal has been generated.

Referring next to FIG. 15, the valid program address (VPA) circuitry 157A shown in FIG. 1 produces a signal VPA as an output on conductor 204, which is wire bonded to lead 7 of the package shown in FIG. 20A. A sum-of-minterm transfer signal produced in sum-of-minterm decoding circuitry 116B (FIGS. 1 and 2) produces a signal that is synchronized with the clock signal .phi.4D and is gated by a "hold program incrementer" signal (HLDPI)*, latched by .phi.4 and ultimately produced on conductor 204 when the present address being output on address bus 101 (FIG. 4) is a program address, i.e., an address of a location in a slow memory, such as a read-only memory or a disc. The valid program address signal on conductor 204 can also caused to be a "zero" in response to the (HLDPI)* or forced break (FRCBRKL) forced break condition due to a hardware interrupt.

The two metal options designated by reference numeral 205 disable the VPA output on conductor 204, if the chip 100 is packaged in the pin compatible 6502 package of FIG. 20B. Referring now to FIG. 16, the valid data address (VDA) circuitry 164 of FIG. 1 is shown in detail. The VDA signal is produced on conductor 206. It is generated as a result of a sum-of-minterm transfer signal (VDA)* produced on conductor 207 by sum-of-minterm decoding circuitry 116B (FIGS. 1, 2, and 4). After being latched by the clock signal .phi.4 and ANDed with the reset signal, and further latched in synchronization with the signal .phi.2D and inverted, the VDA signal is produced on conductor 206. The circuitry 208 in FIG. 16 shows how the signal .phi.2O is derived from .phi.2I when the ready (RDY)* signal produced in block 166 of FIG. 1 add FIG. 4 is latched to a low level. This circuit is similar to the VPA circuit of FIG. 16, except that the VDA signal is at a logical "one" when the address presently being produced on address bus 101 is the location of data (rather than program instructions) stored in a fast memory, and enables an external circuit such as a memory controller to generate a high speed clock that is suitable for accessing a fast RAM. This is more convenient than use of the valid memory address (VMA) produced by some other microprocessors, which do not distinguish between slow program memory and high speed data memory.

Referring now to FIG. 17, the vector pull circuitry of block 165 of FIG. 1 is shown. This circuit produces a logical "zero" on conductor 209 during the two cycles when a vector address is being output on address bus 101. This enables a system using the microprocessor 100 to conveniently modify and/or prioritize interrupt routines, rather than using an entire separate interrupt priority allocation circuit Whenever an interrupt or abort operation occurs, any vector interrupt system, including the one in chip 100, causes vector transfer signals 210 to be applied to vector control circuitry such as 211 that forces a particular vector address into the address latches 121 and 120 of FIG. 4. Those skilled in the art can easily provide input devices which, when enabled by the vector transfer signals 210, force logical "ones" and/or "zeros" into the inputs of the appropriate bits of the address buffers 120 and 121 to load the appropriate vector address therein. Vector control circuitry 211 simply generates a vector pull (VP) signal on conductor 212 whenever this happens. This signal is synchronized with the clock .phi.20 and latched into a latch 165A, inverted, and produced on conductor 209.

Referring now to FIG. 18, new circuitry 166 is shown which produces the signal RDY as a low output level on conductor 213 in response to a wait (WAIT) command applied to one of the inputs of NAND gate 214. This signal is buffered by two cascaded inverters and is applied to an N-channel MOS transistor 215, the drain of which is connected to conductor 213 The IRQ and NMI signals are conventional, but the wait instruction (WAIT) described in Appendix A simply pulls the RDY signal low. This provides an advantage of a simple implementation of the wait for interrupt function The RDY signal on conductor 213 can also be an input which is loaded into latch circuitry 215 in synchronization with the .phi.2I signal, except during a write operation when the chip 100 is emulating a 6502 microprocessor to produce the (RDYL)* signal mentioned above.

The occurrence of either an IRQ or NMI signal disables the wait instruction, allowing conductor 213 to return to its normal high state. (Is this correct, Bill?)

Referring next to FIG. 19, the new M/X output logic circuitry 117A of FIG. 1 is shown. This circuitry simply provides a multiplexed output signal M/X on conductor 216 that, during .phi.2 high time indicates the value of the M bit in status register 117 and during .phi.2 low time indicates the value of the X bit in status register 117. This signal may be useful to external circuitry, such as a memory controller, enabling it to anticipate the operation about to be performed by the incoming op code. The X and M outputs of status register 117 are synchronized by the clock and applied directly, after appropriate inversion and buffering, to conductor 216. The X and M outputs of status register 17 are also transferred in response to an LIR (load instruction register) command into two instruction register latches 217 and 218 to produce the signals and complements routed into the minterm decoding circuitry 116A and also to mode select decode circuitry 160.

In accordance with the present invention, the sixteen bit microprocessor chip 100 executes eight bit op codes, rather than sixteen bit op codes. Most prior sixteen bit microprocessors execute sixteen bit op codes. The eight bit op codes include, as a subset, all of the eight bit op codes of the prior 6502 microprocessor. This enables the sixteen bit microprocessor chip 100 to execute software written for the eight bit 6502 microprocessor. The M bit, the X bit and the E bit can be thought of as "op code extensions" which extend the eight bit op code to effectuate execution of a larger number of instructions having eight bit op codes in either an eight bit or a sixteen bit mode, i.e., wherein the internal data words are eight bits wide or sixteen bits wide, respectively. The way in which this is done can be illustrated with an example of how an LDA (load accumulator) instruction is executed. The op codes for the LDA instruction is A5. For the initial part of this example, assume that M is a "one", indicating that sixteen bit operation is desired. The eight bit op code A5 is loaded into the instruction register, along with E, M, and X, and these are decoded in minterm decoding read-only memory 116A to produce minterm signals which are inverted by minterm inverters 116C. Further decoding then is done on the inverted minterm signals by the sum-of-minterm read-only memory 116B to produce appropriate sum-of-minterm signals. The "one" level of the M bit is then used in the mode select decode circuitry 160 to "select" the appropriate sum of minterm signal as a transfer signal for causing the highest order bit (i.e., bit 7) of the low order byte on the internal data bus 141 (FIG. 4) to be loaded into the N bit of status register 117, since for eight bit operation, the highest order bit on an eight bit data bus would be loaded into the N (negative) bit of the status register for certain instructions.

However, if the M bit is a "zero" instead of a "one", indicating that sixteen bit operation is desired, the operation is similar in that the same op code A5 for the LDA instruction is located into the instruction register and is decoded, with E, M, and X, in the minterm decoding read-only memory 116A and sum-of minterm decoding circuitry 116B, but this time the complement of the M bit, i.e., M*, is used to transfer the highest order bit of the upper byte of the internal data bus 141, i.e., bit 15, into the N bit of the status register 117, since for sixteen bit operation this is the bit that must be loaded into the N (negative) bit of the status register for the same instructions to indicate whether the number on the data bus is a positive number or a negative number.

The above processes can be further understood by reference to the flowchart of FIG. 21, wherein the 16 bit microprocessor first sets the E bit in the status register to a "1" as indicated in block 250. Then the 16 bit microprocessor sets the memory width bit and the register width bit to "1's" in the status register in response to the "1" level of the E bit, as indicated in block 251. Next, the 16 bit microprocessor loads an 8 bit op code into the 8 bit instruction register 118, wherein that op code is one of a set of 8 bit op codes that includes all of the op codes of a 6502 type microprocessor, and also downloads the values of the emulation bit E=1 into the E bit 178A instruction register 118, as indicated in block 252. Then the microprocessor decodes that 8 bit on code and the emulation bit in the instruction register to produce a first group of transfer signals that are useful for effectuating 8 bit information transfers between the various registers and buses of the microprocessor, as indicated in block 253.

Next, the microprocessor further decodes certain of the first group transfer signals and the memory width bit and the register width bit to produce a second group of transfer signals that effectuate 8 bit information transfers between various buses and registers and ALU's of the microprocessor, as indicated in block 254. This causes the 16 bit microprocessor chip to emulate the 8 bit 6502 when the E bit is a "1" by causing it to execute the same set of 8 bit instructions or op codes as the 6502 microprocessor, as indicated in block 255.

If the user desires to operate the 16 bit microprocessor in its 16 bit mode, he causes the E bit to be set to zero, as indicated in block 256. This causes the memory width and register width bits to be set to "0's", as indicated in block 257, and the same 8 bit op code is located in the instruction register and the emulation bit E="0" is loaded into the instruction register, as indicated in block 258. Then the contents of the instruction register are decoded to produce another first group of transfer signals, as indicated in block 259. Then certain transfer signals of that first group are decoded with the memory width and register width bit to produce another second group of transfer signals, as indicated in block 260. The 16 bit microprocessor then is operated to effectuate 16 bit, rather than 8 bit, transfers between the various buses and registers of the microprocessor, as indicated in block 261.

The above-described sixteen bit microprocessor chip 100 provides a number of advantages that are expected to result in a "breakthrough" in microprocessor chip design by providing very high speed sixteen bit internal operation with a number of new featured that greatly simplify the design of low cost computerized systems and allow the sixteen bit microprocessor to easily execute software that has been previously written for the eight bit 6502 microprocessor, and even allows the sixteen bit microprocessor chip to emulate the prior 6502 microprocessor, and achieves this result in a high speed, low power CMOS chip that require less than half as much chip area as prior sixteen bit microprocessors which utilize the generally more area consuming NMOS (N channel MOS) manufacturing technology. The sixteen bit microprocessor chip 100 provides three convenient ways of stopping the clock, by executing a wait instruction, by pulling the "RDY" input to a low level, or executing a "stop the clock" instruction. The provision of simple output circuitry that produces the VPA (valid program address) and VMA (valid memory address) signals allows very convenient use of these signals to determine memory clock speeds in accordance with whether data addresses or program addresses are presently being output, and hence whether high speed data or scratch memory is required or low speed program memory is being addressed. The prior approach would have required execution of a subroutine to produce data indicative of the type of memory being addressed, and decoding of that data by means of external circuitry in order to produce the external signals necessary to accomplish the same result. The vector pull (VP) signal can in some cases simplify vector prioritizing schemes. The abort circuitry allows retention of all data stored in registers accessable by the programmer to be retained, so that at the end of an abort operation, it is not necessary to go all the way back to the beginning of a subroutine in order to complete execution of the subroutine.

While the invention has been described with reference to a particular embodiment thereof, those skilled in the art will be able to make various modifications to the described embodiment of the invention without departing from the true spirit scope thereof. It is intended that operating steps and structural features which are equivalent to those in the described embodiment in that they perform substantially the same work in substantially the same way to achieve substantially the same results are within the scope of the invention. For example, a 32 bit microprocessor may be designed according to the same principles, and could execute the same 8 bit op codes as the above described microprocessor chip 100.

Claims

1. A method of operating a 2.sup.N bit microprocessor comprising the steps of:

(a) setting an emulation bit in a status register in the 2.sup.N bit microprocessor to a first logical level;

(b) setting levels of a memory width bit and a register width bit in the status register to first logic levels in response to the first logic level of said emulation bit;

(c) loading a 2.sup.N-M bit op code in an instruction register, said op code being one of a set of 2.sup.N-M bit op codes that includes all of the op codes of a different 2.sup.N-M bit microprocessor, and also loading the emulation bit into the instruction register;

(d) decoding the 2.sup.N-M bit op code and the emulation bit in the instruction register to produce a first group of transfer signals that are useful for effectuating 2.sup.N bit information transfers between various registers and buses of said 2.sup.N bit microprocessor; and

(e) further decoding certain ones of said first group transfer signals and said memory width bit and said register width bit to produce a second group of transfer signals that are useful in effectuating 2.sup.N-M bit information transfers between various buses and registers and arithmetic logic units of said 2.sup.N bit microprocessor,

said first and second groups of transfer signals causing said 2.sup.N bit microprocessor to emulate said 2.sup.N-M bit microprocessor when said emulation bit has said first level by executing a same set of instructions as said 2.sup.N-M bit microprocessor, wherein N and M are integers and N is greater than M.

2. The method of claim 1 including the steps of:

i. setting said emulation bit to a second logic level;

ii. setting said memory width bit and said register width bits to second logic levels;

iii. repeating steps (c) and (d);

iv. further decoding certain ones of said first group of transfer signals and said memory width bit and said register width bit to produce said second group of transfer signals such that said second group of transfer signals are useful in effectuating 2.sup.N bit information transfers between said various buses and registers and arithmetic logic units of said chip to effectuate full 2.sup.N bit internal operation of said 2.sup.N bit microprocessor.

3. The method of claim 2 wherein said 2.sup.N-M bit op code is one not included in the instruction set of said 2.sup.N-M bit microprocessor.

4. The method of claim 2 wherein said 2.sup.N-M bit op code is one not included in the instruction set of said 2.sup.N bit microprocessor when said 2.sup.N bit microprocessor is operating to effectuate full 2.sup.N bit internal operation.

5. The method of claim 2 wherein N is equal to 4 and M is equal to 1.

6. A 2.sup.N bit microprocessor comprising in combination:

(a) a status register;

(b) means for setting an emulation bit in the status register to a first logical level;

(c) means for setting a memory width bit and a register width bit in the status register to a first logic level in response to the first logic level of said emulation bit;

(d) an instruction register;

(e) means for loading a 2.sup.N-M bit op code in the instruction register, said op code being one of a set of 2.sup.N-M bit op codes that includes all of the op codes of a different 2.sup.N-M bit microprocessor, and also loading the emulation bit into the instruction register;

(f) means for decoding the 2.sup.N-N bit op code and the emulation bit in the instruction register to produce a first group of transfer signals that are useful for effectuating 2.sup.N-M bit information transfers between various registers and buses of said 2.sup.N bit microprocessor; and

(g) means for further decoding certain ones of said first group of transfer signals and said memory width bit and said register width bit to produce a second group of transfer signals that are useful in effectuating 2.sup.N-M bit information transfers between various buses and registers and arithmetic logic units of said 2.sup.N bit microprocessor,

said first and second groups of transfer signals causing said 2.sup.N bit microprocessor to emulate said 2.sup.N-M bit microprocessor when said emulation bit has its first logic level by executing a same set of instructions as said 2.sup.N-M bit microprocessor, wherein N and M are integers and N is greater than M.

7. The 2.sup.N bit microprocessor of claim 6 including:

i. means for setting said emulation bit to a second logic level;

ii. means for setting said memory width bit and said register width bits to second logic levels;

iii. means for further decoding certain ones of said first group of transfer signals and said memory width bit and said register width bit to produce said second group of transfer signals such that said second group of transfer signals is useful in effectuating 2.sup.N bit information transfers between said various buses and registers and arithmetic logic units of said chip to effectuate full 2.sup.N bit internal operation of said 2.sup.N bit microprocessor.

8. The 2.sup.N bit microprocessor of claim 7 wherein said 2.sup.N-M bit op code is one not included in the instruction set of said 2.sup.N-M bit microprocessor.

9. The 2.sup.N bit microprocessor of claim 7 wherein said 2.sup.N-M bit op code is one not included in the instruction set of said 2.sup.N to effectuate full 2.sup.N bit internal operation.

10. The 2.sup.N bit microprocessor of claim 7 wherein N is equal to 4 and M is equal to 1.